CHOLMOD: Fail early when trying to build CHOLMOD_CUDA without OpenMP.

Building CHOLMOD_CUDA requires OpenMP. Instead of failing later on
during compilation, emit an error during configuration that point to the
underlying issue.

Author: mmuetzel

CHOLMOD/CMakeLists.txt

@@ -573,15 +573,9 @@ endif ( )
 
 # CHOLMOD_CUDA
 if ( CHOLMOD_HAS_CUDA )
-#   cmake now configures cholmod.h with "#define CHOLMOD_HAS_CUDA" if CHOLMOD is
-#   being compiled with CUDA, so the -DCHOLMOD_HAS_CUDA flag is no longer needed.
-#   if ( BUILD_SHARED_LIBS )
-#       target_compile_definitions ( CHOLMOD PUBLIC "CHOLMOD_HAS_CUDA" )
-#   endif ( )
-#   set ( CHOLMOD_CFLAGS "${CHOLMOD_CFLAGS} -DCHOLMOD_HAS_CUDA" )
-#   if ( BUILD_STATIC_LIBS )
-#       target_compile_definitions ( CHOLMOD_static PUBLIC "CHOLMOD_HAS_CUDA" )
-#   endif ( )
+    if ( NOT CHOLMOD_HAS_OPENMP )
+      message ( FATAL_ERROR "CHOLMOD_CUDA requires OpenMP. But it has been disabled, or no working OpenMP could be found." )
+    endif ()
     if ( BUILD_SHARED_LIBS )
         target_link_libraries ( CHOLMOD PRIVATE CUDA::nvrtc CUDA::cudart_static CUDA::cublas )
         target_include_directories ( CHOLMOD INTERFACE